U.S. Pat. No. 7,201,120 B2 discloses a single-cylinder two-stroke internal combustion engine mounted on a portable working machine. A carburetor is incorporated in this type of two-stroke engine.
A basic structure and an operation of a general carburetor will be described with reference to FIGS. 18 to 20. The carburetor is defined as follows: “liquid fuel is sprayed in a mist into an intake air passage within the carburetor by using Bernoulli's principle, thereby generating an air-fuel mixture in which the mist fuel and intake air are mixed”. The carburetor includes a venturi portion in order to utilize the Bernoulli's principle.
As is well known, the venturi portion means a structure in which the intake air passage within the carburetor is constricted in an intermediate region of the intake air passage. When the intake air passes through the venturi portion, a flow velocity of the intake air increases. When the flow velocity increases, a static pressure of the intake air decreases, so that the liquid fuel is drawn out into the intake air passage. According to the theory, in the conventional carburetor, a port or a nozzle that feeds the fuel to the intake air passage is positioned in the vicinity of a top portion of the venturi portion.
Referring to FIGS. 18 to 20, an arrow in the drawings indicates a flow of intake air of an engine. A carburetor 100 includes an intake air passage 102, and air filtered by an air cleaner (not shown) passes through the intake air passage 102. The intake air passage 102 includes a venturi portion 104. A throttle valve 106 is arranged downstream of the venturi portion 104, and a choke valve 108 is arranged upstream of the venturi portion 104 in the intake air passage 102. Each of the throttle valve 106 and the choke valve 108 is composed of a butterfly valve. The butterfly valve has a disk shape.
The carburetor 100 has a main system that feeds fuel to the intake air passage 102 in a partial operation (a partial load range) and a high-speed operation (a high load range), and an idle system that feeds fuel to the intake air passage 102 in a low-speed operation such as an idle operation. The main system is also called a throttle system. The idle system is also called a slow system.
The carburetor 100 in the drawings includes a main port 110 of the main system, and a slow port 112 of the slow system. The main port 110 is positioned at a top portion of the venturi portion 104. The slow port 112 is positioned in the vicinity of a peripheral edge of the throttle valve 106 at a fully-closed position of the throttle valve 106. The slow port 112 is composed of first to third idle ports 112(1) to 112(3).
The first idle port 112(1) is called a “primary idle port”. The first idle port 112(1) is located on a downstream side of an air flow direction. The third idle port 112(3) is located on an upstream side of the air flow direction. The second idle port 112(2) is positioned between the first idle port 112(1) and the third idle port 112(3).
FIG. 18 shows a state of the carburetor in the idle operation. In the idle operation, the throttle valve 106 is at a fully-closed position. In this state, the fuel is fed from the first idle port 112(1).
FIG. 19 shows a state of the carburetor in the partial operation. In the partial operation, the throttle valve 106 is in a partially-open state. In this state, the fuel is fed from the main port 110 as well as from the first to third idle ports 112(1) to 112(3).
FIG. 20 shows a state of the carburetor in the high-speed operation. In the high-speed operation, the throttle valve 106 is in a fully-open state. The high-speed operation is called a “full throttle” operation. In the high-speed operation, the fuel is fed from the first to third idle ports 112(1) to 112(3) and the main port 110 similarly to the above partial operation. In the high-speed operation, a large amount of fuel is fed to the intake air passage 102. Therefore, in the high-speed operation, the fuel fed to the intake air passage 102 is mainly fed from the main port 110 located at the top portion of the venturi portion 104.
U.S. Pat. No. 7,100,551 B2 discloses a carburetor incorporated in a stratified scavenging two-stroke engine. The stratified scavenging two-stroke internal combustion engine is described in detail in Japanese Patent Laid-Open No. 2002-227653 and International Publication No. WO 98/57053.
In the stratified scavenging two-stroke engine, scavenging is performed by introducing leading air into a combustion chamber at an initial stage of a scavenging stroke and subsequently introducing an air-fuel mixture into the combustion chamber in order to reduce a blow-by phenomenon of an unburnt gas occurring at the initial stage of the scavenging stroke. In the carburetor incorporated in the stratified scavenging engine, a fresh air passage that feeds air filtered by an air cleaner to an engine body, and an air-fuel mixture passage that generates an air-fuel mixture and feeds the air-fuel mixture to the engine body (a crankcase) are formed when a throttle valve is in a fully-open state. The fresh air passage is connected to a scavenging passage that communicates with the crankcase and the combustion chamber.
In the stratified scavenging engine, it is desirable to fill an upper portion of the scavenging passage with the fresh air in an amount equivalent to the blow-by of the unburnt gas, and also to fill the crankcase of the engine body with the whole air-fuel mixture generated in the carburetor in a state of a full throttle (a high-speed operation).
The carburetor disclosed in U.S. Pat. No. 7,100,551 B2 includes a dividing wall within the carburetor arranged on an upstream side of the throttle valve. The dividing wall within the carburetor divides an intake air passage on the upstream side of the throttle valve into a first passage and a second passage. That is, each of the first and second passages divided by the dividing wall independently extends to the vicinity of the throttle valve from an upstream end of the carburetor.
When the throttle valve is in a fully-open state, the throttle valve becomes adjacent to the dividing wall within the carburetor, and the first passage and the second passage are thereby extended to a downstream end of the carburetor by the throttle valve in the high-speed operation (the full throttle).
In the full throttle, i.e., the high-speed operation, the fresh air filtered by the air cleaner is fed to the engine body through the first passage.
In the carburetor disclosed in U.S. Pat. No. 7,100,551 B2, a main port located at a top portion of a venturi portion is located facing the dividing wall within the carburetor. The second passage constitutes the air-fuel mixture passage, and the air-fuel mixture is fed to the engine body (the crankcase).
FIG. 21 shows a schematic diagram of the carburetor disclosed in U.S. Pat. No. 7,201,120 B2. Referring to FIG. 21, a carburetor 200 of U.S. Pat. No. 7,201,120 B2 includes a main nozzle 202. Reference numeral 204 denotes a throttle valve, and reference numeral 206 denotes a venturi portion. Note that reference numeral 208 denotes an air cleaner.
The main nozzle 202 is arranged in the vicinity of a top portion of the venturi portion 206, and is positioned in a state inclined toward a plate surface of the throttle valve 204 in a fully-open state. That is, the main nozzle 202 is arranged so as to be inclined toward a downstream side. On the other hand, a protrusion portion 212 is formed at an upstream end portion of the throttle valve 204, and protrudes into an air-fuel mixture passage so as to approach the main nozzle. The protrusion portion 212 is provided for the purpose of increasing a flow velocity of intake air indicated by an arrow by constricting the air-fuel mixture passage.
Reference numeral 220 shown in FIG. 21 denotes an intake member. The carburetor 200 is connected to an engine body 214 by the intake member 220. The intake member 220 includes a fresh air passage 224 and an air-fuel mixture passage 226 formed by a dividing wall 222. The air-fuel mixture passage 226 communicates with a crankcase 216 of the engine body 214.
In a full throttle, i.e., a high-speed operation, fuel discharged from the main nozzle 202 is received by the throttle valve 204, and is entirely fed to the crankcase 216 of the engine body 214. In FIG. 21, reference numeral 218 denotes a piston, and reference numeral 228 denotes a combustion chamber.